This invention is based upon the discovery of certain materials which act as catalysts to increase the rate at which organic materials can be burned out in essentially non-oxidizing atmospheres, i.e., atmospheres containing very low levels of oxygen, e.g., &lt;25 ppm oxygen. One particular application for the invention contemplates the use of the catalysts in the fabrication of microelectronic circuits prepared through thick-film technology. Most advantageously, the catalysts are employed in the fabrication of such circuits where copper metal constitutes the thick-film conductor.
In the most general terms, the preparation of thick-film circuits comprises applying a paste, commonly termed an ink, containing a conducting material onto a ceramic substrate by means of a screen or mask ("printing") and then firing the coated substrate to adhere the paste to the substrate. Because of its excellent insulating properties and stability, and relatively high thermal conductivity and strength, alumina (Al.sub.2 O.sub.3) has been widely used in the fabrication of such substrates. Nevertheless, because of the relatively high dielectric constant of Al.sub.2 O.sub.3 (.apprxeq.10), substrates prepared therefrom are subject to significant signal propagation delays and noise when used in certain high performance applications. Moreover and very importantly, the high maturing or sintering temperatures of Al.sub.2 O.sub.3 (.apprxeq.1600.degree. C.) severely limit the selection of co-sinterable conducting metallurgies to highly refractory metals such as molybdenum, palladium, platinum, and tungsten. That is, metals such as copper, gold, and silver which exhibit much greater electrical conductivity cannot be used because their melting points are far below 1600.degree. C.
Those intrinsic drawbacks of Al.sub.2 O.sub.3 have led to the development of glass-ceramic substrates. Such bodies are prepared from glass powders which are sintered together and exhibit the capability of crystallizing in situ during the sintering firing. Thermally devitrifiable glass compositions have been developed which can be sintered into non-porous bodies and crystalized in situ at atmospheres below 1000.degree. C. Examples of such compositions can be found in U.S. Pat. No. 4,234,367 and U.S. Pat. No. 4,301,324, wherein the predominant crystal phase developed is either .beta.-spodumene solid solution of cordierite, and in U.S. application Ser. No. 923,432, filed Oct. 27, 1986, U.S. Pat. No 4,714,687, in the names of Louis M. Holleran and Francis W. Martin, wherein the predominant crystal phase developed is willemite with, optionally, cordierite.
Customarily, the fabrication of microelectronic circuits contemplates interconnected multilayer substrates consisting of sintered glass-ceramic insulator and conducting patterns made of thick-film gold, silver, or copper. Interconnections between buried conductor layers can be achieved through vias comprising metal paste-filled holes in the individual laminae formed prior to lamination which will, upon sintering, become densely sintered metal interconnections. The production of such multilayer glass-ceramic substrates commonly involves the following eight general steps:
(a) Glass of a chosen composition is comminuted to a very fine powder, typically to particle sizes less than 10 microns.
(b) A slurry of that powder is prepared by admixing the powder in an organic vehicle system consisting, in general, of a binder, e.g., polyvinyl butyral, solvents, e.g., toluene and ethanol, and a surfactant, e.g., phosphate ester. Steps (a) and (b) may be combined, if desired, by milling the glass in the organic vehicle.
(c) The slurry is cast into thin green sheets in accordance with conventional techniques, for example, by doctor blading.
(d) The cast sheets are cut to desired dimensions and vias formed therein in predetermined configurations.
(e) A metallizing paste of gold, silver, or copper is deposited into the vias of the sheets, for example, by screen printing.
(f) The desired conductor patterns are applied onto the sheets; again, for example, by screen printing.
(g) A plurality of the sheets are laminated together in registry. The temperature and pressure employed for lamination are designed to cause the individual green sheets to bond together into an integral monolithic green substrate, and to cause the green ceramic to flow adequately to enclose the conductor patterns.
(g) The laminated substrate is fired to a temperature and for a time sufficient to burn out the organic components, and the temperature then raised to cause the sintering together of the glass particles with their concurrent conversion into glass-ceramic bodies through crystallization in situ, and the sintering together of the metal particles in the conductor patterns to dense metal lines and vias.
Silver and gold conducting pastes can be fired in an air atmosphere. Nevertheless, because of the characteristic tendency of silver to diffuse into the glass-ceramic and its apparent proclivity to cause electromigration problems, and because of the extremely high cost of gold, copper-containing pastes have customarily constituted the economic choice. Unfortunately, however, copper exhibits a high oxidizing potential, which attribute requires multilayer structures containing copper pastes to be sintered in neutral or reducing environments. Disadvantageously, reducing atmospheres have been observed to occasion adhesion problems; accordingly, neutral environments have been deemed preferable. Nonetheless, although operable in preventing the oxidation of copper, a neutral atmosphere, e.g., nitrogen, argon, and/or helium, does not provide an oxidizing agent capable of removing the carbon-based (organic) vehicle efficiently. That is, in the absence of an oxidizing agent, the organic components pyrolyze as the substrates are fired.
This situation has led to numerous schemes for controlling the atmosphere by limiting the oxygen content thereof. For example, each of U.S. Pat. Nos. 3,726,006, 4,296,272, 4,311,730, 4,313,262, and 4,517,155 demonstrates the use of oxygen in restricted amounts to oxidize the volatilized constituents of the organic vehicle system and remove them from the burnout chamber, or the use of small amounts of oxygen to produce a resistor by oxidizing the material deposited on the substrate. Still other attempts to solve the burnout problem inherent in the use of inert atmospheres through control of the components thereof or through control of the composition of the thick film are illustrated in U.S. Pat. Nos. 4,409,261 and 4,122,232. Finally, U.S. Pat. No. 4,622,240 discloses the use of an atmosphere of nitrogen containing 10-10,000 ppm nitrous oxide (N.sub.2 O) in the burnout environment.
The period of time required to burn out the organic vehicle system in copper-containing, thick-film atmosphere in a burnout chamber. Self-evidently, then, a reduction in firing time would result in a significant cost saving.
Therefore, the principal objective of the present invention was to devise means for accelerating the rate of burnout of organic materials in a gaseous, essentially non-oxidizing atmosphere. Most specifically, the instant invention was designed to provide means for increasing the rate of burnout of the organic components from multilayer microelectronic circuit packages utilizing glass-ceramic substrates and copper-containing pastes when such packages are fired in a gaseous, essentially non-oxidizing atmosphere.